UPX packing and unpacking

[mMhCkB]

I dont understand why you would need upx to pack the exe back to the exact original size but anyways, it is possible that it was packed using something like UPX-Scrambler or another variation on upx, thus, the output is slightly different from that of a standard upx compression.

Regarding making a upx file not unpackable, while i suppose it would be quite easy to modify it so upx wouldn't recognize the signatures, bratalarms generic upx unpacker would always be able to handle it, or if you're into manual unpacking, it would still be easy.

If you have any more questions, feel free to ask me :)
mM

[disavowed]

1)I have an exe packed by UPX, I unpacked it, then packed again, but
alas, the size of the file was different.

if you patched it between the time you unpacked it and re-packed it, then the compression algorithm will result in different outputs of different sizes.